Synopsis

Description

BEGIN initiates a transaction block, that is, all statements after a
BEGIN command will be executed in a single transaction until an
explicit COMMIT or ROLLBACK is given. By default
(without BEGIN), Greenplum Database executes transactions in
autocommit mode, that is, each statement is executed in its own transaction and a
commit is implicitly performed at the end of the statement (if execution was
successful, otherwise a rollback is done).

Statements are executed more quickly in a transaction block, because transaction
start/commit requires significant CPU and disk activity. Execution of multiple
statements inside a transaction is also useful to ensure consistency when making
several related changes: other sessions will be unable to see the intermediate
states wherein not all the related updates have been done.

If the isolation level or read/write mode is specified, the new transaction has those
characteristics, as if SET TRANSACTION was executed.

Parameters

WORK

TRANSACTION

Optional key words. They have no effect.

SERIALIZABLE

READ COMMITTED

READ UNCOMMITTED

The SQL standard defines four transaction isolation levels:
READ COMMITTED, READ UNCOMMITTED,
SERIALIZABLE, and REPEATABLE READ. The
default behavior is that a statement can only see rows committed before it
began (READ COMMITTED). In Greenplum Database READ
UNCOMMITTED is treated the same as READ
COMMITTED. REPEATABLE READ is not supported;
use SERIALIZABLE if this behavior is required.
SERIALIZABLE is the strictest transaction isolation.
This level emulates serial transaction execution, as if transactions had
been executed one after another, serially, rather than concurrently.
Applications using this level must be prepared to retry transactions due to
serialization failures.

READ WRITE

READ ONLY

Determines whether the transaction is read/write or read-only. Read/write is
the default. When a transaction is read-only, the following SQL commands are
disallowed: INSERT, UPDATE,
DELETE, and COPY FROM if the table
they would write to is not a temporary table; all CREATE,
ALTER, and DROP commands;
GRANT, REVOKE,
TRUNCATE; and EXPLAIN ANALYZE and
EXECUTE if the command they would execute is among
those listed.

Notes

Issuing BEGIN when already inside a transaction block will provoke a
warning message. The state of the transaction is not affected. To nest transactions
within a transaction block, use savepoints (see SAVEPOINT).

Examples

To begin a transaction block:

BEGIN;

To begin a transaction block with the serializable isolation level:

BEGIN TRANSACTION ISOLATION LEVEL SERIALIZABLE;

Compatibility

BEGIN is a Greenplum Database language extension. It is equivalent
to the SQL-standard command START TRANSACTION.

Incidentally, the BEGIN key word is used for a different purpose in
embedded SQL. You are advised to be careful about the transaction semantics when
porting database applications.